Hi,

there are no special instructions for the comparator. Portpin 1.0 is IN+ and Portpin 1.1 is IN- of the comparator. The output of the comparator is internally connected to the Portpin 3.6. If you want to know the status of the Portpin 3.6 you can simply use the Port read instruction
( JNZ P3.6, Label ; DJNZ P3.6 ....)

The question is not " are there instructions for the comparator". The question is: How can i use the comparator at best? Therefore you should have a look at the atmel homepage. There you'll find the application note:

Analog-to-Digital Conversation Utilizing the AT89CX051 Microcontroller

http://www.atmel.com/atmel/products/prod72.htm

Bye, Dieter.